Mass spectrometry is an analytical methodology used for qualitative and quantitative analysis of compounds in a chemical or biological sample. Analytes in a sample are ionized, separated according to their mass by a spectrometer and detected to produce a mass spectrum. The mass spectrum provides information about the masses and in some cases the quantities of the various analytes that make up the sample. In particular embodiments, mass spectrometry can be used to determine the molecular weight or the molecular structure of an analyte in a sample. Because mass spectrometry is fast, specific and sensitive, mass spectrometer devices have been widely used for the rapid identification and characterization of biological analytes.
Mass spectrometers may be configured in many different ways, but are generally distinguishable by the ionization methods employed and the ion separation methods employed. For example, in certain devices parent analyte ions are isolated, the parent ions are fragmented to produce daughter ions and the daughter ions are subjected to mass analysis. The identity and/or structure of the parent analyte ion can be deduced from the masses of the daughter ions. Such devices, generally referred to as tandem mass spectrometers (or MS/MS devices) may be coupled with a liquid chromatography system (e.g., an HPLC system or the like) and a suitable ion source (e.g. an electrospray ion source) to investigate analytes in a liquid sample.
In certain mass spectrometers, e.g., so-called single quadrupole (“single quad”), triple quadrupole (“triple quad”), Q-TOF and Qq-TOF mass spectrometers, a quadrupole mass filter is employed. For example, in a single quadrupole mass spectrometer, an ion stream passes through the ion passageway of a quadrupole mass filter, and ions of a particular m/z (mass to charge ratio) are selected. The selected ions are detected by a detector that is present at the ion exit end of the ion passageway. In a triple quadrupole mass spectrometer, an ion stream passes through the ion passageway of a first quadrupole mass filter, and ions of a particular m/z ratio (mass to charge ratio) are selected. The selected ions are then fragmented in a collision cell to produce daughter ions, and those daughter ions are passed through a second quadrupole mass filter where daughter ions of a particular m/z ratio are selected. The selected daughter ions are detected by a detector that is present at the ion exit of the second mass filter. Because single and triple quadrupole mass spectrometers are relatively inexpensive and very robust, they are widely employed for sample analysis.
However, despite their popularity, mass spectrometers that employ a quadrupole mass filter are limited in their sensitivity because a significant number of ions are lost from the spectrometer between certain stages of the spectrometer (e.g., between a quadrupole mass filter and a collision cell or between a quadrupole mass filter and the detector). This is because ions typically have a significant amount of radial energy when they exit a quadrupole mass filter, and their flight trajectory is not towards to the next stage. This problem is exacerbated as the mass of ions gets larger, and critical when analyzing low abundance ions having a mass of greater than about 0.5 kDa.
There is therefore a need for new mass spectrometers in which ions are efficiently transferred out of a quadrupole mass filter into the next stage. This invention meets this need, and others.